1. Technical Field
The present invention relates to a sputtering target suitable for forming a oxide thin film and a process for producing thereof, wherein the oxide thin film is composed of an oxide sintered body which comprises indium oxide having a bixbyite structure, and a Yb2Fe3O7 structure compound represented by the formula: In2Ga2ZnO7.
The present invention also relates to a sputtering target suitable for forming a oxide semi-conductor and a method for producing thereof, wherein the oxide semi-conductor comprises oxides of indium (In), gallium (Ga) and zinc (Zn), and is composed of an oxide sintered body which comprises indium oxide having a bixbyite structure and a homologous structure compound InGaO3(ZnO)m (m is a natural number of 1 to 4).
The present invention relates to a sintered body having a crystal structure of rare-earth oxide C type.
The present invention relates to a target having a crystal structure of rare-earth oxide C type, especially relates to a target suitable for forming an amorphous oxide film by sputtering and a method for producing thereof.
2. Background of the Art
An oxide semi-conductor film composed of a metallic composite oxide includes, for example, an oxide semi-conductor film composed of oxides of In, Ga and Zn (IGZO) (hereinafter also referred to as “oxide thin film”). An oxide semi-conductor film which film is formed by IGZO sputtering target draws attention as one having mobility greater than that of an amorphous Si film. Such an oxide semi-conductor film has mobility and visible light permeability greater than that of an amorphous Si film, and thus the film also draws attention as one to be applied for a switching element (thin film transistor) such as a liquid crystal display device or a thin film electroluminescence display device.
The IGZO sputtering target is known as one containing a compound represented by InGaO3(ZnO)m (m is a natural number of 1 to 20) as a main component. However, when a sputtering (DC sputtering) is conducted by using the IGZO sputtering target, an abnormal electrical discharge caused by anomalous growth of the compound shown by InGaO3(ZnO)m, and thus there was a problem about defects on the obtained film. In addition, specific resistance of the obtained sputtering target was at least around 1×10−2 Ωcm, this resistance is high and thus a plasma discharge is not stable, and DC sputtering was thus difficult, on top of that, a target which causes cracks during sputtering could not obtained.
A sputtering target intended to form an amorphous oxide film is known (Patent Document 1). In this document, a sintered body indicating crystal structure having homologous phase (InGa(ZnO)m; m is natural number of less than 6) is used, and then the major component thereof is different from that of the present invention. In addition, the specific resistance of the target is around 1×10−2 Ωcm or more which is so high, the target is not appropriate for conducting DC sputtering method having good productivity. The spattering target composed of a compound single phase represented by homologues structure InGaO3(ZnO)m (m is natural number of 1 to 4) disclosed in the Patent Document 1 has different structure compounds of a sintered body from those of the present invention. The production process to obtain the sintered body of the compound single phase is complicated, sintering time is long, and thus a target sintered body having low cost could not be obtained. In addition, sintering conditions for forming a (single phase) sintered body consisting only of this homologues structure InGaO3(ZnO)m are limited. Furthermore, bulk resistance of this homologues structure InGaO3(ZnO)m (m is natural number of 1 to 4) was usually high such as 102 to 103 Ωcm, therefore, the bulk resistance was reduced by conducting reduction treatment after sintering. However, the bulk resistance after the reduction was at most around 1×10−1 Ωcm, and thus effect of reducing the bulk resistance was small compared to a high number of production steps thereof. Further, the sputtering target composed of a sintered body only consisting of a compound represented by the homologues structure InGaO3(ZnO)m causes an abnormal electrical discharge by anomalous growth during film-forming by sputtering, and thus there was a problem about defects on the obtained film.
In addition, a compound represented by In2Ga2ZnO7 or In2Ga2ZnO7-d which has oxygen defect d is known as an oxide sintered body used as an electrode for a liquid crystal display, an EL display and a solar cell (Patent Documents 2 and 3). In this case, electrically conductive is applied by introducing an amount of oxygen defect d to a compound represented by In2Ga2ZnO7, but the inventions of these documents have different crystal structure from that of the present invention. Therefore, in the production steps of an oxide sintered body, it was impossible to shorten a reduction step and to elongate a sintering time, and thus neither productivity could be heightened, nor cost could be lowered.
There is a report by Kimizuka, et. al. concerning a phase diagram of In2O3—Ga2ZnO4—ZnO at 1350° C. However, the invention of this document has different crystal structure from that of the present invention. Therefore, in the production steps of an oxide sintered body, it was impossible to shorten a reduction step and to elongate a sintering time, and thus neither productivity could be heightened, nor cost could be lowered.
A field effect type transistor such as a thin-film transistor (TFT) is widely used for a unit electron device for a semi-conductor memory integrated circuit, a high-frequency signal amplifier device, a device for driving a liquid crystal, and is the most practically used electric device.
Among the above, because of recent dramatic progress in a display device, TFT is heavily used as a switching device for driving a display device by adding drive voltage to a display device among various kind of a display device such as a liquid crystal display device (LCD), an electroluminescence display device (EL), or a field emotion display (FED).
A silicon semi-conductor compound is most widely used as a material for a semi-conductor layer which is a primary member of a field effect type transistor. A silicon single crystal is generally used for a high-frequency signal amplifier device or a device for an integrated circuit where high-speed operation is required. For a device for driving a liquid crystal, an amorphous silicon semi-conductor (amorphous silicon) is used because of a request for one having large-area.
For example, as the TFT, there is an opposite stagger structure which laminates a gate electrode, a gate insulation layer, a semi-conductor layer such as a hydrogenated amorphous silicon (a-Si:H), a source electrode and drain electrode on a substrate such as a glass. This TFT is used for an image sensor, as well as a driving device for a flat panel display represented by an active matrix type liquid crystal display, within a field of a large-area device. Among these usages, high-speed driving is required based on high-functionalization, even though conventional amorphous silicon is used.
Currently, a switching device for driving a display device mainly uses a device using a silicon semi-conductor film, because a silicon thin film has various good properties such as stability, good workability, and high switching speed. This silicon thin film is generally prepared by a chemical vapor deposition (CVD) method.
A crystalline silicon thin film requires, for example, high temperature of 800° C. or more during crystallization, and thus it is difficult to compose it on a glass substrate or an organic material substrate. Therefore, there are several problems, for example, the film can be formed only on an expensive substrate having high heat resistance such as a silicon wafer or quartz, and the production thereof requires a lot of energy and many steps.
A crystalline silicon thin film is limited to a top-gate structure for a devise structure of an ordinary TFT, and thus it is difficult to reduce costs by cutting a number of masks down.
An amorphous silicon thin film can form under comparatively low temperature. However, switching speed of the film is slower than that of the crystalline film, and therefore, when the film is used for a switching device for driving a display device, displaying high-speed motion picture cannot be achieved sometimes.
Specifically, a liquid crystal television having VGA definition can use an amorphous silicon having 0.5 to 1 cm2/Vs in mobility. But if the definition becomes SXGA, UXGA, QXGA, or higher, the mobility requires 2 cm2/Vs or more. If driving frequency is raised for enhancing image quality, the mobility also must become higher.
An organic light emitting display is driven by current. Therefore, if an amorphous silicon which varies its properties by DC stress is used, there is a problem where image quality is degraded based on its long-term use.
In addition, if a crystalline silicon is used for the above usages, it becomes difficult to apply it to large-area. There is also a problem where production cost becomes higher, because high temperature heating treatment is needed.
Under these situations, recently, an oxide semi-conductor thin film using oxides is coming up as one having excellent stability rather than that of a silicon semi-conductor thin film
For example, TFT using zinc oxide as a semi-conductor layer is released.
However, this semi-conductor layer has low mobility: about 1 cm2/V·sec, and low ON-OFF ratio. In addition, leak current is easy to occur, and thus it was difficult to put it into industrial, practical use. Furthermore, many attempts were taken place for an oxide semi-conductor containing crystalline material using zinc oxide, but the following problems were present when a film was formed by a sputtering method which is commercially and commonly used.
That is, mobility is low, ON-OFF ratio is low, leak current is high, pinch-off is unclear, normally-ON is easy to occur, and thus properties in TFT are lowered. The obtained film has less chemical resistance, and thus process or usage environment is limited, for example, wet-etching is difficult. Further, a film must be formed under high pressure in order to improve its properties, but film-forming speed is slow as well as 700° C. or more of high temperature is required, and thus there are problems in industrialization. In addition, TFT properties such as mobility in a bottom-gate structure is low, and thus a TFT device structure is also limited, for example, top-gate structure and 50 nm or more of thickness are required for raising its properties.
In order to solve these problems, a method for producing an amorphous oxide semi-conductor film composed of indium oxide, zinc oxide and gallium oxide so as to drive a thin film transistor is discussed.
For instance, a target composed of a sintered body indicating a homologous crystal structure comprising indium oxide, zinc oxide and gallium oxide is disclosed (Patent Documents 4 and 5). However, the homologous crystal structure has less heat stability, and thus crystalline morphology is varied based on small change of sintering temperature or sintering time. Therefore, there are problems concerning instability in properties such as density in a target, bulk resistance, bending strength and surface roughness. In addition, if the target is used as one for making a thin film transistor, there are problems particularly for a homologous crystal structure where properties of the transistor is significantly varied between at the start of forming a film and at the end of forming a film.                A target composed of indium oxide, zinc oxide, and gallium oxide having 2.2 to 40 atom % of Ga, 50 to 90 atom % of In and 95% or more of relative density is disclosed (Patent Document 6). However, there is no discussion about a target having less than 50 atom % of In.        
It is known that In2O3 (indium oxide) has a crystal structure of rare-earth oxide C type, and having high mobility caused by the crystal structure of rare-earth oxide C type. However, it has been difficult to make a sintered body which can readily incorporate oxygen at the time of sintering to have low resistance. A sputtering target composed of In2O3 (indium oxide) or containing plenty of In2O3 (indium oxide) has problems concerning that nodules (agglomerates found on a surface of the sputtering target) are easily created, that there are many particles (dusts of a sputter material found during sputtering), and that an abnormal electrical discharge during sputtering is easily occurred. It is known that if In2O3 (indium oxide) contains an atom other than In, a crystalline type other than rare-earth oxide C type such as β-GaInO3, β-Ga2O3, ZnGa2O4 is formed. Especially, it is also known that if Ga2O3 is contained in an amount of 10 mass % or more (In2O3 becomes 90 mass % or less), β-Ga2O3 is formed (Non-Patent Document 1). If a crystalline type other than rare-earth oxide C type, especially β-Ga2O3 is formed, problems such as crack being formed, bulk resistance being high, relative density being low, bending strength (JIS R1601) being low, difficulty in obtaining a sputtering target being uniformity in various physical properties such as a structure, are easily occurred, and thus it becomes not appropriate for applying it to a sputtering target for industrial use. If it is used for forming a semi-conductor film, there are possibilities to cause ununiformity parts so as not to obtain in-plain uniformity, to cause reduction in yield ratio, or to cause reduction in reliability (stability).    Patent Document 1: JP-A-2007-73312    Patent Document 2: JP-B-3947575    Patent Document 3: JP-B-3644647    Patent Document 4: JP-A-2000-044236    Patent Document 5: JP-A-2007-73312    Patent Document 6: JP-A-H10-63429    Patent Document 7: JP-A-2007-223849    Non-Patent Document 1; Journal of the American Ceramic Society 1997, 80, pp. 253 to 257